Heterostructures composed of two‐dimensional black phosphorus (2D BP) with unique physical/chemical properties are of great interest. Herein, we report a simple solvothermal method to synthesize in‐plane BP/Co2P heterostructures for electrocatalysis. By using the reactive edge defects of the BP nanosheets as the initial sites, Co2P nanocrystals are selectively grown on the BP edges to form the in‐plane BP/Co2P heterostructures. Owing to disposition on the original defects of BP, Co2P improves the conductivity and offers more active electrocatalytic sites, so that the BP/Co2P nanosheets exhibit better and more stable electrocatalytic activities in the hydrogen evolution and oxygen evolution reactions. Our work not only extends the application of BP to electrochemistry, but also provides a new idea to improve the performance of BP by utilization of defects. Furthermore, this strategy can be extended to produce other BP heterostructures to expand the corresponding applications. In‐plane black phosphorus (BP)/Co2P heterostructures were synthesized using the reactive edge defects of the BP nanosheets as the initial sites. Co2P selectively grown on the BP edges improves the conductivity and offers more active electrocatalytic sites. The BP/Co2P nanosheets exhibit a better and more stable electrocatalytic activity in hydrogen and oxygen evolution reactions.